1. Technical Field of the Invention
This invention relates generally to electromagnetic transducers such as audio speakers, and more specifically to suspension components such as spiders, and to means for centering the voice coil axially and radially.
2. Background Art
FIG. 1 illustrates a conventional speaker 10 with an external magnet geometry motor structure 12 driving its diaphragm assembly 14. The motor structure includes a soft magnetic T-yoke 16 or pole which includes a back plate 18 and a pole piece 20 that are either magnetically coupled or of integral construction. The T-yoke may optionally include a ventilation hole 22 for depressurizing the diaphragm assembly. One or more external ring permanent magnets 24 are magnetically coupled to the back plate. A soft magnetic top plate 26 is magnetically coupled to the permanent magnets. A magnetic air gap 28 is formed between the top plate and the pole piece.
The diaphragm assembly includes a flexible suspension component 34 known as a surround, a diaphragm 32 or cone serving as the principal acoustic element, a voice coil former or bobbin 36 coupled to an electrically conductive voice coil 40, a spider 38 which acts as a second suspension component, and a dust dome 42 attached to the diaphragm to seal the open end of the bobbin and serving as a secondary acoustic element. A frame or basket 30 is mechanically attached to the motor assembly and supports the suspension components of the diaphragm assembly. The surround and spider allow the bobbin and diaphragm to move axially with respect to the motor structure but prevent, as much as possible, their lateral or radial movement. The voice coil is wound around and mechanically coupled to the bobbin, and is disposed within the magnetic air gap of the motor structure. The spider and the surround must keep the voice coil and the bobbin from rubbing against any part of the stationary motor structure.
FIG. 2 illustrates a conventional speaker 50 with an internal magnet geometry motor structure 52 driving the diaphragm assembly 14. The motor structure includes a soft magnetic yoke 54 or cup. One or more internal permanent magnets 56 are magnetically coupled to the yoke, and an internal soft magnetic top plate 58 is magnetically coupled to the permanent magnets, forming a magnetic air gap 60 between the top plate and the yoke. The motor structure may be ventilated, as shown, or it may be unventilated and have disc magnets and a disc plate, rather than the ring configuration shown.
To achieve the long axial excursions required to produce low frequencies, it is desirable that the suspension components provide as much radial centering force as possible, but as little axial force as possible. It is also desirable that the suspension components have as little mass as possible, and as little unit-to-unit variability as possible, so a production run of speakers will have predictable, constant characteristics of resonant frequency, frequency response, efficiency, and so forth.
FIG. 3 illustrates a conventional spider 38. One aspect of a conventional spider whose unit-to-unit process variability is undesirably high, is the glue which is used to couple the inner diameter 62 of the spider to the bobbin (not shown). It is difficult to control the precise amount of glue applied to each unit, which results in slightly different moving mass from speaker to speaker. Furthermore, when the glue is applied to the flexible spider, the glue tends to spread outward from the bobbin, wicking into the material of the spider to an outer glue perimeter 64. The glued portion will typically be stiffer than the rest of the spider, increasing the spider's overall stiffness. Unit-to-unit variance in the distance that the glue wicks will result in higher speaker-to-speaker performance variability.
FIG. 4 illustrates a further complication that results from gluing the spider 38 to the bobbin (not shown). If the glue wicks to a perimeter 66 which is asymmetric in shape or which is asymmetrical about the axis of the bobbin, the suspension will be asymmetrical. Mass and stiffness asymmetries will tend to induce rocking of the moving parts, causing collisions against the non-moving parts. Such collisions not only produce unpleasant noise, but also reduce the performance of the speaker and may even damage it.
Yet another disadvantage is present in the prior art. During the assembly process, labor-intensive, time-consuming, and expensive steps and equipment are used in order to ensure that the moving parts are radially and axially centered about the non-moving parts of the motor structure. Complex assembly fixtures must be employed in expensive, automated assembly lines to meet minimum process repeatability requirements.
Another disadvantage of the prior art is the relatively low excursion enabled in typical small speakers, with their resulting low sound pressure levels and poor low frequency performance. One notable improvement in small speakers is illustrated in PCT patent application PCT/US99/15962 published as WO 00/05925 “Miniature Full Range Loudspeaker” by inventor Clayton Williamson. Williamson's speaker uses a conventional motor and a conventional diaphragm assembly, but attaches the surround to the bobbin (“voice coil form”) rather than to the diaphragm and at a point somewhat lower than the outer end of the bobbin. Although the application is somewhat silent on this particular topic, having the surround attached at this more centralized point should tend to reduce rocking a little, although, because of its use of only a single suspension component, significantly less than the rocking reduction achieved by the present invention.
U.S. Pat. No. 5,081,684 “Shallow Loudspeaker with Slotted Magnet Structure” by William N. House teaches a speaker motor structure having a slotted yoke. The diaphragm is external to the radial dimensions of the yoke, and is rigidly coupled to the bobbin by ribs which extend through the slots. The sole purpose of the slotted motor structure is to allow a shallower overall speaker by allowing the rigid attachment ribs to pass within the motor structure, such that the diaphragm may be substantially coplanar with the motor structure rather than being positioned out in front of the motor structure as is conventionally the case.